Rechargeable electrochemical cell

ABSTRACT

A rechargeable electrochemical battery cell with a housing, a positive electrode, a negative electrode and an electrolyte which contains SO 2  and a conducting salt of the active metal of the cell, whereby at least one of the electrodes contains a binder chosen from the group: Binder A, which consists of a polymer, which is made of monomeric structural units of a conjugated carboxylic acid or of the alkali salt, earth alkali salt or ammonium salt of this conjugated carboxylic acid or a combination thereof or binder B which consists of a polymer based on monomeric styrene structural units or butadiene structural units or a mixture of binder A and B.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/496,517, filed Sep. 25, 2014, which claims priority to DE 10 2013 016560.1, filed Sep. 27, 2013, both of which are hereby incorporated hereinby reference in their entireties.

BACKGROUND

The invention relates to a rechargeable lithium battery cell with apositive electrode, a negative electrode and an electrolyte thatcontains a conducting salt.

Rechargeable battery cells are of great importance in many technicalfields. They are often used for applications in which only relativelysmall current levels are required, such as mobile phones, power toolsand other mobile applications. There is also a great demand for batterycells for high current applications (high-current cells), where theelectric propulsion of vehicles, the application as local energy storageor the application as mass storage for grid stabilization (gridstorage/grid tie application/smart grid) are of particular importance.

Development aims of rechargeable battery cells are particularly a highenergy density (electrical energy per unit weight and volume), highextractable currents (low internal resistance), long service life, inparticular a large number of useful charging and discharging cycles,very good operational safety and costs kept to a minimum.

Rechargeable lithium cells are in practice almost exclusivelylithium-ion cells. Their negative electrode consists of copper-coatedcarbon, in which lithium-ions are stored during charging. The positiveelectrode also consists of an insertion material that is suitable forabsorbing ions of the active metal. Normally the positive electrode isbased on lithium-cobalt oxide which is coated onto an aluminumconducting element. Both electrodes are very thin (thickness typicallyless than 100 m). In order to stabilize the electrodes in a mechanicalway, binder is used in addition to the active material. During charging,the ions of the active metal are discharged from the positive electrodeand inserted into the negative electrode. During discharging the reverseprocess occurs. The ions are transported between the electrodes by meansof the electrolyte, which has the required ion mobility. Lithium-ioncells contain an electrolyte consisting of a lithium salt (e.g., LiPF₆)dissolved in an organic solvent or a solvent mixture (e.g., based onethylene carbonate). They are also designated hereafter as “organiclithium-ion cells.”

Organic lithium-ion cells are problematic with regard to safety. Safetyrisks are caused in particular by the organic electrolyte. If alithium-ion cell catches fire or even explodes, the organic solvent ofthe electrolyte forms the combustible material. In order to avoid suchhazards, additional measures must be taken, in particular with regard toa very precise regulation of the charging and discharging processes andwith regard to additional safety measures in the battery design. Forexample, the cell contains components that melt in the event of a faultand therefore prevent the flow of current in the cell. However, thesemeasures lead to increased costs and increased volume and weight, thusreducing the energy density. Furthermore these methods are not alwayssufficiently and safety risks with above described effects can occur.

The problems are particularly serious when battery cells are to bedeveloped for mass storage applications, as required for newlydeveloping markets. The requirements on the stability and long-termoperational safety are particularly high.

There is a high demand for improved rechargeable battery cells, which inparticular meet the following requirements:

-   -   Very good electrical performance figures, in particular high        energy density combined with high current draw values (power        density).    -   Safety, even under the more challenging operating conditions in        a vehicle, the application as local energy storage or the        application as mass storage for grid stabilization (grid        storage/grid tie application/smart grid).    -   Long service life, in particular a high number of usable charge        and discharge cycles.    -   Use of cost-effective materials.    -   Cost-effective and maximally simple production methods.    -   Further important practical requirements, such as overload        capability and deep discharge capability.

In WO 2011/098233 A2, a battery cell is described that satisfies thesepartially conflicting demands in a substantially better manner than hasbeen the case up to now. It is characterized by the following specialfeatures, which are also preferably implemented in the battery cell ofthe present invention:

-   a) The electrolyte contains SO₂. Preferably this is a SO₂ based    electrolyte. This is the term used in the context of the invention    to designate an electrolyte which contains SO₂ not merely as an    additive at a low concentration, but in which the concentration of    the SO₂ is so high that the mobility of the ions in the conducting    salt, which is contained in the electrolyte and causes the charge    transport, is at least in part guaranteed by the SO₂. The    electrolyte is preferably substantially free of organic materials,    where “substantially” is to be understood in the sense that the    quantity of any organic materials that may be present is so low that    they do not present a safety hazard.-   b) The positive electrode contains an active material having the    composition Li_(x)M′_(y)M″_(z)(XO₄)_(a)F_(b), wherein    -   M′ is at least one metal selected from the group consisting of        the elements Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn,    -   M″ is at least one metal selected from the group consisting of        the metals of groups 2, 3, 4, 5, 6, 8, 9, 10, 11, 12, 13, 14, 15        and 16 of the periodic table,    -   X is selected from the group consisting of the elements P, Si        and S,    -   x is greater than 0,    -   y is greater than 0,    -   z is greater than or equal to 0,    -   a is greater than 0 and    -   b is greater than or equal to 0.

The element X here is preferably P. Particularly preferably M′ is themetal Fe and particularly preferably b is equal to 0. Lithium-ironphosphates are particularly preferred active materials of the positiveelectrode, for example active materials as LiFePO₄,Li_(x)Fe_(y)M_(z)PO₄, Li_(x)Fe_(y)(SO₄)_(a) or LiFeSO₄F, where thesuffixes x, y, z and a have the above interpretation.

The active material may contain an additional doping, which is not acomponent part of its atomic structure.

On the basis of this prior art the invention aims to create a cell withimproved functioning and operational safety.

This technical problem is solved by a rechargeable electrochemicalbattery cell comprising a housing, a positive electrode, a negativeelectrode and an electrolyte that contains SO₂ and a conducting salt ofthe active metal of the cell, wherein at least one electrode contains abinder selected from the group consisting of

binder A,

-   -   consists of a polymer, build up    -   from monomeric structural units of a conjugated carboxylic acid    -   or from the alkali salt, alkaline earth salt or ammonium salt of        the conjugated carboxylic acid,    -   or from a combination of the above

or binder B

-   -   consists of a polymer, based on    -   monomeric styrene or butadiene structural units

or a mixture of binder A and binder B

Examples for binder A are lithium polyacrylate (Li-PAA) or polyacrylicacid.

Examples for binder B, which is based on monomeric styrene structuralunits and butadiene structural units, are the products TRD102A andTRD2001 by JSR Micro.

A mixture of binder A and B is considered to be a mixture of thedescribed polymers or a mixture of the monomeric structural units of therespective binder.

For instance, a polymer consisting of the monomeric structural units ofstyrene, butadiene and acrylic acid is a mixture of the individualmonomeric structural units.

By combining the features according to the invention a substantialimprovement of the function of the cells is achieved. In particular itwas found within the context of the invention that the amount of lithiumions which are irreversibly used for the solid electrolyte interfaceformation on the negative electrode during the first charge cycle issubstantially reduced. Consequently the battery cell has more cycleablecapacity left for subsequent cycles.

The cycle life of the cell is extended by the increased initialcapacity. In addition, the mechanical and chemical stability of thenegative and the positive electrode is improved by the use of binder Aor B or a mixture of A and B. This leads to an extended cycle life ofthe battery cell, too.

The production process of the electrodes and their processing in thefurther process of the production of cells are substantially simplified.As binder A and binder B are water soluble or dispersible in water allproduction steps can be taken without the use of toxic, polluting andhighly flammable organic solvents. There is no need for special safetymeasures (such as ventilation, sensor monitoring or explosionprotection), enclosed production devices or elaborate solvent recovery.

The electrodes consist each of an active material, that changes theoxidation state during charging or discharging of the cell. Charging anddischarging is intercalating and deintercalating ions of the activemetals of the cell, especially lithium ions. Electrons released or usedin this process move into an electronically conductive current collectorwhich is part of the electrode as well.

As already mentioned, the present invention is preferably used in arechargeable lithium battery cell in accordance with WO 2011/098233 A2.Reference is made to the content of this document in full. For example,the present invention incorporates the following special features thatare substantially described in the document cited, from which furtherdetails can also be obtained:

-   -   The positive electrode is unusually thick, minimum thicknesses        of 0.25 mm, 0.3 mm, 0.4 mm, 0.5 mm, 0.6 mm, 0.8 mm and 1.0 mm        being particularly preferred, in this order. Preferably, the        maximum thickness of the positive electrode is 2 mm,        particularly preferably a maximum of 1.5 mm.    -   The negative electrode preferably contains carbon as the active        material for absorbing lithium ions. It is also preferably        unusually thick, minimum thicknesses of 0.2 mm, 0.3 mm, 0.4 mm,        0.5 mm, 0.6 mm and 0.8 mm being particularly preferred, in this        order. The thickness of the negative electrode is preferably a        maximum of 1.5 mm, particularly preferably a maximum of 1 mm.    -   The electrodes have a conducting element with a        three-dimensional porous metal structure, in particular in the        form of a metal foam. It is particularly preferred if this        porous metal structure extends substantially over the entire        thickness of the electrode. In accordance with a further        preferred embodiment the active material of the electrode is        substantially uniformly distributed in the three-dimensional        porous metal structure that forms the conducting element of said        electrode.    -   The electrolyte contains a relatively high concentration of SO₂        in comparison to the amount of conducting salt, with minimum        values of 1.5 mol SO₂, 2 mol SO₂, 2.5 mol SO₂, 3 mol SO₂, 4 mol        SO₂, 4.5 mol SO₂, 5 mol SO₂ and 6 mol SO₂ per mol conducting        salt being particularly preferred in this order. Preferably, the        maximum concentration is 22 mol SO₂ per mol conducting salt.    -   The electrolyte contains as a conductive salt a halogenides,        oxalates, borates, phosphates, arsenates and gallates of the        active metal. Preferably a lithium tetrahalogenoaluminate is        used and particularly preferably a lithium tetrachloroaluminate.

The solution to the problem addressed by the invention was faced with anumber of difficult issues related to the use of an inorganic,preferably 50₂-based, electrolyte and the use of thick electrodes, witha conducting element with a three-dimensional porous metal structure,which are not present in the case of conventional cells with an organicelectrolyte and thin layer electrodes.

-   -   The electrodes contain a current collector with a three        dimensional porous metal structure particularly in the form of        metal foam. In order to evenly distribute the active material        together with the binder within the three dimensional porous        metal structure, a homogenous mixture of the components together        with a solvent needs to be produced. This mixture must be easy        to introduce into the metal structure. If these conditions are        not met, substantial problems arise in the production of a thick        electrode.    -   Binders, such as fluorinated binders, dissolve often only in        highly flammable organic solvents, which are detrimental to the        environment. The production of binder-containing electrodes        requires use of elaborate equipment, which takes into account        the use of these solvents. Explosion prevention, environmental        protection and the protection of the exposed employees are        especially problematic during electrode production.    -   The good electric connection of the active material to the three        dimensional porous metal structure must not be hindered by the        binder.    -   The optimal content of binder is difficult to determine:    -   A binder content in the electrodes, which is too low, leads to        difficult handling of the produced electrodes, as the almost        binder-free electrodes have too little adhesion to the current        collector. Upon inappropriate operation and under unfavorable        ambient conditions there may occur a release of particles of the        active material. This can lead to uselessness of the product,        soiling of the work area and putting employees at risk through        uncontrolled uptake of small particles.    -   A binder content, which is too high, has a negative effect on        the energy density of the battery cell. The weight of the binder        reduces the energy density.    -   The electrolyte is very corrosive. The binder must be stable        towards the electrolyte containing SO₂. Thus, the choice of        suitable material is very limited.    -   The binder must maintain its stability over a long period of        time, even if in case of malfunctioning during the charge and        discharge cycles the active metal—lithium in case of a lithium        cell—is deposited in its metallic state and gets into contact        with the binder. If the binder reacts with the metal, a        destabilization of the mechanical structure of the electrode        will be the consequence. The cell becomes useless.    -   The formation of surface layers on the negative electrode in        electrolytes containing SO₂ is increased by the use of binders        such as fluorinated binders, especially THV (terpolymer from        tetrafluorethylene, hexafluoropropylene an vinylidene fluoride)        and polyvinylidenefluoride (PVDF). The cyclable capacity in the        following cycles is due to this loss reduced.    -   Binders in the electrode often lead to a poor wettability of the        surface of the electrode with an electrolyte containing SO₂.        This leads to high impedance of the cell, leading to problems        during the operating of the cell.    -   Because of the use of binders, the internal resistance R_(i)        increases as the electronic connection of the active material to        the three dimensional metal structure is reduced, so that a        compromise between the mechanical stabilization and the increase        of the resistance must be found.

In the course of the invention it was found, that, despite theseconcerns, the use of a binder A or B or a mixture of A and B in theelectrodes, and especially in the negative electrode, surprisingly isnot only possibly but also especially advantageous. The advantages ofthe battery cell relating to the invention containing these electrodesare the increased initial capacity as well as the chemical andmechanical stabilization of the electrodes, as explained above. Inaddition, the production was simplified.

Further advantages are achieved in consideration of the preferredembodiment, described below.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned aspects of exemplary embodiments will become moreapparent and will be better understood by reference to the followingdescription of the embodiments taken in conjunction with theaccompanying drawings, wherein:

FIG. 1 a cross-section model of a battery cell according to theinvention;

FIG. 2 Drawing in perspective of an electrode with schematicallyenlarged image details for the explanation of their inner structure;

FIG. 3 the graph of the electric potential over the charge capacity forfour negative electrodes with different binder materials;

FIG. 4 the graph of the electric potential over the charge capacity forthree different negative electrodes;

FIG. 5 the graph of the electric potential over the charge capacity forfour negative electrodes with a different content of Li-PAA binder; and

FIG. 6 the comparison of the absolute level of dischargeable capacity ofcells with thermal pretreatment of the negative electrode and a negativeelectrode, which has been produced with a binder according to theinvention.

DESCRIPTION

The embodiments described below are not intended to be exhaustive or tolimit the invention to the precise forms disclosed in the followingdetailed description. Rather, the embodiments are chosen and describedso that others skilled in the art may appreciate and understand theprinciples and practices of this disclosure.

The housing 1 of the rechargeable battery cell 2 shown in FIG. 1encloses an electrode arrangement 3 comprising a plurality (three in thecase shown) of positive electrodes 4 and a plurality (four in the caseshown) of negative electrodes 5. The electrodes 4, 5 are connected inthe usual manner with corresponding terminal contacts 9, 10 of thebattery by means of electrode leads 6, 7. The cell is filled with anSO₂-based electrolyte, not shown in the figures, in such a manner thatthe electrolyte preferably penetrates completely into all pores,particularly inside the electrodes 4, 5.

As is common, the electrodes 4, 5 have a planar shape, i.e., they areshaped as layers having a thickness which is small relative to theirextension in the other two dimensions. They are separated from eachother by separators 11. The housing 1 of the prismatic cell shown isessentially cuboid, the electrodes and the walls shown in cross-sectionin FIG. 1 extending perpendicularly to the drawing plane and beingessentially straight and flat. However, the cell according to theinvention can also be designed as a spirally wound cell.

The electrodes 4, 5 comprise in usual manner a current collectorelement, which is made of metal and serves to provide the requiredelectronically conductive connection of the active material of therespective electrode. The current collector element is in contact withthe active material involved in the electrode reaction of the respectiveelectrode. Preferably, the current collector element of the positiveelectrode, most preferably also the current collector element of thenegative electrode, is provided in the form of a three-dimensionalporous metal structure, particularly in the form of a metal foam. Theterm “three-dimensional porous metal structure” designates in thiscontext every structure made of metal that does not just extend like athin sheet only over the length and width of the planar electrode, butalso extends over its thickness dimension, and which is porous in such amanner that the active material of the electrode can be incorporatedinto the pores.

FIG. 4 shows, by means of two schematic enlarged excerpts, the internalstructure of a preferred positive electrode. It has a conducting element30 with a three-dimensional porous metal structure. The conductingelement is preferably formed by a metal foam, wherein it is particularlyadvantageous if the porous metal structure extends substantially overthe entire thickness d of the positive electrode. The active material 33of the electrode, for example for a positive electrode lithium-ironphosphate, is located in the pores of the porous metal structure and ispreferably distributed homogeneously therein. Further details can beobtained from the above-mentioned document WO 2011/098233 A2. In thecontext of the invention particularly advantageous results have beenfound to be obtained with the combination of the electrode typesdescribed there and a binder A or B or a mixture of A and B as describedhere.

During manufacture of the electrode, the active material and binder A orB or a mixture of A and B are incorporated into the porous structure ofthe current collector element such that it fills the pores of thecurrent collector element uniformly over the whole thickness of themetal structure. The material is then pressed under high pressure, thethickness after the pressing operation being preferably no more than80%, particularly preferably no more than 60% and more particularlypreferably no more than 40% of the initial thickness.

The three-dimensional porous metal structure 30 of the current collectorelement extends essentially over the whole thickness d of the currentcollector element and the active material and binder A or B or a mixtureof A and B are distributed essentially homogeneously therein. Withrespect to the two stated conditions, “essentially” is to be construedsuch that the cell function is only slightly impaired by any deviations.In any case, the porous metal structure should extend over at least 70%,preferably at least approximately 80%, of the thickness of theelectrode.

A preferred embodiment of the battery cell according to the inventionhas positive and negative electrodes of which at least either thepositive or the negative electrodes contain only binder A.

Another preferred embodiment of the battery cell according to theinvention has positive and negative electrodes of which at least eitherthe positive or the negative electrodes contain only binder B.

Another preferred embodiment of the battery cell according to theinvention has positive and negative electrodes of which at least eitherthe positive or the negative electrodes contain a mixture of binder Aand binder B.

Another preferred embodiment of the battery cell according to theinvention has positive and negative electrodes of which at least eitherthe positive or the negative electrodes contain a binder chosen from thegroup:

Binder A which consists of a polymer which is made of monomericstructural units of a conjugated carboxylic acid or of the alkali saltor earth alkali salt or ammonium salt of this conjugated carboxylic acidor of a combination of these

or binder B which consists of a polymer based on monomeric styrenestructural units or butadiene structural units

or a mixture of binder A and binder B

and which in addition contain another binder which is different frombinder A and binder B.

The electrodes described in WO 2011/098233 A2 are remarkably thick. Dueto the thickness and, in addition due to the pore structure of the usedporous metal structure of the current collector, there were additionalproblems expected in combination with a binder A or binder B or amixture of A and B.

In order to receive a high percentage of solid material in theelectrode, the paste used in production must have optimalcharacteristics, the paste is made of active material, binder, possiblyfurther components and solvent. Only then the pores of the currentcollector consisting of a porous metallic structure can be filled almostcompletely with solid material and so an electrode with high capacitycan be manufactured.

The mechanical and chemical stability of the negative and the positiveelectrode are important criteria for the quality of a rechargeablebattery cell. This stability can be obtained by the use of a binder inthe electrodes. A binder must meet the following requirements for anelectrode:

-   -   Simple processibility upon production of the electrode:    -   homogeneous mixing with the other parts of the electrode such as        the active material to obtain an electrode with sufficient        loading.    -   Good solubility or dispersibility in the solvent, which should        be water, because of its easy handling.    -   Suitable melting range to avoid both melting away (e.g.,        stability during thermal drying of the cell) in the further        process and being too firm (e.g., process temperature during        activation of the binder).    -   No release of harmful gases (e.g., gases containing fluorine)        upon thermal stress.    -   High shelf life (e.g., storage temperature and storage period).    -   Positive characteristics upon operation of the battery cell.    -   No degradation of the binder by the electrolyte. Chemical        compatibility with all cell components.    -   Enabling the formation of a thinner surface layer on the        electrodes especially on the negative electrode.    -   Mechanical stabilization of the electrode over a long period of        time or during many charge and discharge cycles. This mainly        serves the purpose to compensate for the volume changes during        intercalation and de-intercalation.    -   Good wettability of the surface of the electrode with        electrolyte.    -   Thermal stability within the operating temperature of the cell.

As the cell according to the invention preferably has three dimensionalcurrent collectors and a SO₂ containing electrolyte, the selection of abinder meeting all of the above mentioned requirements is particularlydifficult.

Example 1

In the state of the art, as for instance described in WO 2011/098233 A2,the filling of the metal structure of the positive electrode wasachieved by means of an organic solvent containing a soluble fluorinecontaining binder. The obtained capacities were typically around 15mAh/cm² of electrode surface area.

During the invention, it was tried to simplify the production of thepositive electrodes through the substitution of the organic solvent bywater. Therefore the fluorine containing binder was dispersed in water.It was found, that the achievable loadings were reduced by around 7% toapprox. 14 mAh/cm².

Surprisingly, the filling of the metal structure could in fact beoptimized with one of the binders A or B according to the invention or amixture of A and B and water as solvent, so that a similarly high levelof filling of active material can be reached as previously only possiblewith the of use organic solvents.

As an example, for a preferred embodiment of the battery cell accordingto the invention positive electrodes were produced from the followingcomponents:

94 wt % lithium iron phosphate (active material of the positiveelectrode)

2 wt % carbon black (conductivity enhancer)

4 wt % binder A

With stirring, these components were used to produce a paste usingwater. The finished paste was introduced homogeneously into a metal foamwith an initial porosity of more than 90% and dried at 50° C. for onehour. This step is necessary in order to obtain solvent free electrodes.Through calandering technology, after cooling the electrode wascompressed from an initial thickness of approx. 1.00 mm to 0.56 mm,followed by another drying process at 120° C. in vacuum. The achievedcapacities of these positive electrodes are typically again at 15mAh/cm² of electrode surface area.

With a negative electrode a similar filling level was achieved throughthe optimization of the production parameters with water as solvent, aspreviously only achieved with the use of organic solvents.

As an example, for the production of a preferred embodiment of thebattery cell according to the invention, the following components forthe negative electrode were used:

96 wt % graphite (active material of the negative electrode)

4 wt % binder A

Whilst stirring, a paste was produced from the components and water assolvent. The paste was introduced homogeneously into a metal-foam withan initial porosity of more than 90% and dried at 50° C. for one hour.This step is necessary to obtain solvent free electrodes. Throughcalandering technology, after cooling the electrode was compressed froman initial thickness of 1.00 mm to a thickness of 0.40-0.42 mm, followedby another drying process at 120° C. in vacuum.

The achieved capacities of the produced negative electrodes according tothe invention are around 13 mAh/cm² electrode surface are.

Example 2

The battery cell as described in WO 2011/098233 A2 contains a negativeelectrode, which is free of binder. This is due to the fact that manycommon binder materials, which can exclusively be used with organic andflammable solvents are not stable towards the used inorganicelectrolyte.

The absence of a binder adds complexity to the production process of thenegative electrode and leads to complex solutions.

Another reason for not using binder is that the addition of the thereindescribed fluorine containing binder leads to a significant rise in theamount of lithium ions, which, during the first charge cycle, areirreversibly used by the formation of the surface layer on the negativeelectrode.

The impact of various binders on the irreversibly used capacity due tothe formation of the surface layer on the negative electrode in thefirst cycle has been examined. To this end, various negative electrodeswere produced with graphite as active material, the corresponding binderand a three dimensional current collector, as described in experiment 1.A binder-free reference electrode was produced the same way. The contentof binder was adjusted to the different binder characteristics of theindividual binder.

It should be noted that a high percentage of binder has a negativeimpact on the energy density respectively on the electrical energy perweight and volume unit of the battery cell.

In addition, a binder-free reference electrode was produced the sameway. Five different sets of experimental electrodes have beenmanufactured this way. Table 1 describes the used binders:

TABLE 1 complete description of the binders used in example 2 electrodeselectrodes with electrodes with binder of state binder free bindersaccording of the art electrodes the invention number in A B C D E FIG. 3wt % of 0, 1 1 0 2 4 binder solvent Aceton Isopropanol IsopropanolWasser Wasser (production) polymer THV PTFE ,— SBR LiPAA Terpolymer ofPolytetra- Styrene- Lithium- Tetrafluorethylene, fluorethylene ButadienePolyacrylate Hexafluorpropylene und Vinylidene fluoride chemical(—CF₂—CF₂— CF₂— (—CF₂—CF₂—)_(n) ,— [—CH₂—CH═CH— [—CH₂— structure C₂F₄—CH₂—CF₂—)_(n) CH₂—]_(m)[—CHPh— CH(COOX)—]_(n) CH₂—|_(n)

The five experimental electrodes were examined via a three-electrodearrangement, where, during the charging of the electrode, the course ofthe electrical potential U expressed in volts was shown over the stateof charge Q in relation to the rated capacity Q_(N) of the electrode.The measurements were run in an electrolyte consisting of LiAlCl₄×1.5SO₂.

The five graphs show the results of several experiments with the abovedescribed electrodes. In FIG. 3 the abscissa of the intercept betweengraph and 400 mV line corresponds to the used cell capacity due toformation of surface layer. It can be seen that the loss of capacity ofthe electrode with fluorine containing binders THV and PTFE is highest,followed by the reference electrode not containing a binder.

The curve of the electrode with the SBR binder according to theinvention shows a significantly better relationship than the previouselectrodes.

The lowest loss of capacity of just 6% has the electrode according tothe invention with Li-PAA binder.

It is clearly visible that the electrode without binder also has a highloss of capacity during formation of the surface layer.

Table 2 summarizes the results:

TABLE 2 Used cell capacity due to formation of a surface layer forelectrodes with different binder. electrodes electrodes with electrodeswith binder of state binder free binders according of the art electrodesthe invention number in FIG. 3 A B C D E Polymer THV PTFE — SBR LiPAAUsed cell capacity 17% 18% 14% 11% 6% due to formation of a surfacelayer

Example 3

In order to reduce the very high capacity of surface layer of abinder-free electrode WO 2011/098233 A2 suggests elaborate measures suchas, e.g., the temperature treatment of electrodes at 900° C. minimum forat least 10 h or the coating of the surface of the electrode. After theproduction of the electrode, both measures require time-consuming andcostly production steps, such as described in example 1.

Electrodes produces according to example 1 can be used in a battery cellaccording to the invention without further treatment.

The test was performed analogous to example 2. In FIG. 4 as well theabscissa of the intercept between curve an 400 mV line corresponds tothe used cell capacity due to formation of surface layer. The followingelectrodes were used:

A. Electrode with temperature treatment according to WO 2011/098233 A2;

B. Electrode with coating according to WO 2011/098233 A2; and

C. Electrode with 4% Li-PAA.

It is clearly visible that the two electrodes A and B have asubstantially higher irreversible capacity loss than electrode C.

Table 3 summarizes the results:

TABLE 3 Irreversible capacity loss due to formation of a surface layerfor electrodes treated differently. electrodes electrode with electrodetemperature electrode with 4% treatment with coating Li-PAA number inFIG. 3 A B C Irreversible capacity 12% 8% 6% loss due to formation of asurface layer

It is clearly visible that the irreversible capacity loss due toformation of a surface layer for electrodes, which are manufacturedaccording to procedure described in WO 2011/098233 A2, has still highvalues.

Surprisingly, an electrode according to the invention shows outstandingbehavior even without work-intensive post-treatment.

Example 4

In example 4, the influence of the content of binder on the irreversiblecapacity loss due to formation of a surface layer was examined.

In order to compare electrodes according to the invention with thecurrent state of the art, negative electrodes with different levels ofTHV binder were produced according to the procedure with acetone assolvent as described in example 1.

Here, the surface layer was determined through charging and dischargingat 1 C rate two times. The cycle efficiency of the first two cycles wasconverted into the irreversible capacity loss due to formation of asurface layer (in % of the theoretical discharge capacity).

Table 4 shows the results:

TABLE 4 Irreversible capacity loss due to formation of a surface layerfor electrodes with different content of THV binder. wt % of THV-binder1 2 4 used cell capacity (in %) of the theoretical 24.1% 26.4% 33.5%capacity, due to formation of a surface layer

It can clearly be seen that a higher level of binder has a negativeimpact on the characteristics of the electrodes. Whereas theirreversible capacity loss due to formation of a surface layer is stillat 24.1% at a binder level of 1 wt % THV, it rises by approx. 10% pointsto 33.5% at a binder level of 4 wt %.

FIG. 5 shows negative electrodes according to the invention withdifferent content of Li-PAA binder. The capacity of the surface layercan be seen in the graph.

Here, surprisingly, better results can be achieved with a higher levelof binder. The performance of the electrodes with binder according tothe invention is contrary to the performance of the electrodes with THVbinder.

The best results are achieved with a level of Li-PAA of 8 wt %. But alsothe electrode with a content of Li-PAA of 2 wt % shows a significantimprovement of the electrode.

However, the electrodes with a high level of Li-PAA show a lower energydensity as the electrodes contain less active material. The increase ofbinder content within an electrode must not exceed the decrease ofirreversible capacity loss due to the replacement of active material,otherwise there is no increase in the further cycleable capacity. Bestresults are achieved with a binder content of approx. 4 wt %.

Example 5

For this experiment, two prismatic full cells with two negative and apositive electrode were produced. The electrodes were stacked togetherwith a separator arranged in between and introduced in a prismatichousing.

Both full cells contained positive electrodes which were produced fromthe following components according to the procedure in Example 1, yetwith acetone as solvent:

94 wt % lithium iron phosphate

2 wt % carbon black (conductivity enhancer)

4 wt % THV as binder

Positive electrodes with a capacity of 15 mAh/cm² were produced.

Full cell B contained binder-free negative electrodes, which weretreated with a temperature treatment according to WO 2011/098233 A2.

For the production of complete cell A negative electrodes according tothe invention were produced from the following components according tothe procedure in Example 1:

96 wt % graphite (active material of the negative electrode)

4 wt % binder A

Negative electrodes with a capacity of 14 mAH/cm² electrode surface wereproduced respectively.

The complete cells were filled with an electrolyte consisting ofLiAlCl₄×6SO₂.

First, the cell was gradually conditioned at a charge and discharge rateof approx. 0.05 C (equals 10 mA). In doing so, the surface layer isformed and the irreversible capacity loss due to the formation of thesurface layer can be determined.

During the following 100 cycles the cells were cycled at a charge anddischarge rate of approx. 0.5 C (equals 100 mA). After 100 cycles thecharge rate and discharge rate is increased to approx. 1 C (equals 200mA).

The charge was carried out in an IU process. The cell is being chargedwith a constant current up to a voltage level of 3.6 V. At a cellvoltage of 3.6 V the current is reduced. When the current first fallsbelow a minimum current of 40 mA the charge process is stopped.

The discharge was carried out with a constant current and stopped at acell voltage of 2.5 V.

FIG. 6 shows the discharge capacity over the number of cycles. The twographs shown indicate average values of various experiments with theelectrodes described above.

It is surprising that despite the addition of 4% binder a higherabsolute discharge capacity can be reached.

The cycle life of a cell is determined by the retaining a certaindischarge capacity, e.g., 70% of the nominal capacity. Due to the higherinitial capacity the cycle life of cell A is substantially higher thanthe one of cell B. Cell A reaches the final discharge capacity laterthan cell B.

While exemplary embodiments have been disclosed hereinabove, the presentinvention is not limited to the disclosed embodiments. Instead, thisapplication is intended to cover any variations, uses, or adaptations ofthis disclosure using its general principles. Further, this applicationis intended to cover such departures from the present disclosure as comewithin known or customary practice in the art to which this inventionpertains and which fall within the limits of the appended claims.

What is claimed is:
 1. A rechargeable electrochemical battery cellhaving a housing, a positive electrode, a negative electrode and anelectrolyte containing SO₂ and a conductive salt of the active metal ofthe cell, wherein at least one electrode contains a binder selected fromthe group consisting of: binder A, wherein binder A comprises a polymer,build up from monomeric structural units of a conjugated carboxylic acidor from the alkali salt, alkaline earth salt or ammonium salt of theconjugated carboxylic acid, or from a combination of the above, binderB, wherein binder B comprises a polymer, based on monomeric styrene orbutadiene structural units, and a mixture of binder A and binder B 2.The battery cell according to claim 1, wherein the electrodes comprisingthe binder are the negatives electrodes of the cell.
 3. The battery cellaccording to claim 1, wherein at least one of the electrodes containsbinder A or B or a mixture of A and B in a concentration of no more than10%.
 4. The battery cell according to claim 1, wherein at least one ofthe electrodes contains additional binder different from binder A or B.5. The battery cell according claim 1, wherein at least one of theelectrodes has a current collector element with a three-dimensionalporous metal structure.
 6. The battery cell according claim 5, whereinthe porous metal structure extends essentially over the entire thicknessof the electrode.
 7. The battery cell according claim 5, wherein theactive metal is distributed essentially homogeneously in the positivemetal structure.
 8. The battery cell according claim 1, wherein thepositive electrode has a thickness of at least 0.25 mm.
 9. The batterycell according claim 1, wherein the positive electrode has a thicknessof at least 0.25 mm and a maximum thickness of 2.0 mm.
 10. The batterycell according claim 1, wherein the negative electrode has a thicknessof at least 0.2 mm.
 11. The battery cell according claim 1, wherein thenegative electrode has a thickness of at least 0.2 mm and a maximumthickness of 1.5 mm.
 12. The battery cell according claim 1, wherein theactive metal is selected from the group consisting of alkali metals,alkaline earth metals, metals in the subgroup 12 of the periodic systemand aluminum.
 13. The battery cell according claim 12, wherein theactive metal is lithium, sodium, calcium, zinc, or aluminum.
 14. Thebattery cell according claim 1, wherein the negative electrode is aninsertion electrode.
 15. The battery cell according claim 14, whereinthe negative electrode contains carbon.
 16. The battery cell accordingclaim 1, wherein the positive electrode contains a metal oxide or ametal halide or a metal phosphate.
 17. The battery cell according claim16, wherein the positive electrode contains an intercalation compound.18. The battery cell according to claim 17, wherein the positiveelectrode contains lithium iron phosphate.
 19. The battery cellaccording claim 1, wherein the electrolyte is based on SO₂ and whereinthe electrolyte contains at least 1.5 mol SO₂ per mol conducting salt.20. The battery cell according claim 1, wherein the electrolyte is basedon SO₂ and wherein the electrolyte contains at least 1.5 mol SO₂ per molconducting salt and the maximum concentration is 22 mol SO₂ per molconducting salt
 21. The battery cell according claim 1, wherein theelectrolyte contains a halide, an oxalate, a borate, a phosphate, anarsenate, or a gallate of the active metal, as the conductive salt.